1. Technical Field
The present invention relates to an organic-inorganic composite forming material which can be used as a material for optical elements such as lenses, diffraction gratings, optical waveguides and light-emitting elements, an organic-inorganic composite and a production method thereof, and an optical element using the organic-inorganic composite.
2. Description of Related Art
Camera-mounted mobile telephones and digital cameras increasingly demand resin-made aspherical lenses or composite aspherical lenses.
The composite aspherical lens is a lens which carries a resin lens having an aspherical shape on a glass lens. Since a thickness of the resin portion is about several hundred micrometers and smaller than that of the resin-made aspherical lens, the composite aspherical lens undergoes a smaller change under the influence of temperature and shows a superior heat resistance compared to the resin-made aspherical lens. The composite aspherical lens in a mobile telephone is required to exhibit a high environment resistance, e.g., withstand a heat at about 150° C. Also, the resin must be photocurable in order to prevent curing shrinkage that may occur during a process where the resin is cured on the glass lens. Even in the case where the composite aspherical lens uses a high-refractive index glass lens, its overall thickness must be reduced in order to reduce the size or thickness of a device. Accordingly, although a choice of a material quality of the glass lens is important, a refractive index of the resin portion itself must also be increased.
There is a method for increasing the index of refraction of the resin by mixing high-refractive, fine oxide particles in the resin. International Publication No. WO 02/088255 discloses an organic-inorganic hybrid polymer material as a heat-resisting resin, which is obtained by incorporating metal oxide particles in an organic polymer having a metal alkoxy group. However, in International Publication No. WO 02/088255, the organic-inorganic hybrid polymer material is synthesized by melt blending. Accordingly, if the composite aspherical lens is to be fabricated using this polymer material, the resin portion must be formed on the glass lens by heat processing. Also, neither the heat resistance required for the composite aspherical lens nor an optical resin having a high refractive index is described.
Japanese Patent Laid-Open No. 2005-298796 proposes an optical resin which uses a resin obtained via hydrolysis and polycondensation of alkoxysilane and contains fine particles of a metal oxide such as niobium oxide. In order to increase a refractive index of the optical resin, such fine particles of metal oxide may need to be incorporated in a large amount. However, no description is provided with regard to a method for dispersing them in a stable fashion. When those fine particles of metal oxide are incorporated in a large amount, they hold together to form agglomerates that scatter a light and, as a result, cause clouding of the resin, which is a problem.